A known coupling yoke of this type is of a so-called split type that includes: a pair of mutually facing bridge members having bearing holes for rotatably supporting one of two shaft portions of each spider at both axial ends; surrounding means that surrounds the axial centers of both bridge members; and fastening means that fastens the bridge members and the surrounding means (for example, see Patent Document 1).
In addition, another coupling yoke is of an integral type that includes: a pair of mutually facing flange portions each of which has a bearing hole for allowing one of shaft portions of each spider to be rotatably installed and that protrude from each of both axial ends of a base portion in the axial direction. The integral-type coupling yoke is molded by casting or forging, and does not require assembling work such that bridge members and surrounding means, are assembled and then these are fastened by fastening means as in the case of the split-type coupling yoke described in Patent Document 1, so it is advantageous in terms of working man-hour. In addition, because it is of an integral type, it is excellent in terms of stiffness.
In the existing integral-type coupling yoke, the base portion from which the flange portions protrude is formed of a block-like thick wall portion; however, in the case where the drive-side yoke and the driven-side yoke, which are coupled by the coupling yoke, are spaced apart slightly, when the length of the base portion is extended in the axial direction in order to ensure torsional stiffness, the overall weight of the coupling yoke increases accordingly. As the weight increases, the inertia moment of the coupling yoke increases, so a loss of energy at the time when driving force is transmitted from the drive-side yoke to the driven-side yoke increases.
Then, in order to suppress an increase in the weight of the base portion, it is conceivable that, as shown in FIG. 20, a depressed area or a recess 112 is formed near the axial center of a base portion 111, and, as shown in FIG. 21, an opening 113 is formed similarly near the axial center of the base portion 111.